A network-based system may create, store, and manage representations of various entities and various relationships between entities. Examples of entities include users (e.g., buyers, sellers, or administrators), products (e.g., items that are specimens of a product), or classifications (e.g., categories, catalogs, groups of users, or groups of products). For example, a product within a category may be available for purchase from a seller, and the seller may seek to merchandise one or more items as instances or specimens of the product. The product may take the form of a good or a service. Examples of goods include physical items (e.g., a digital camera or a car) and information items (e.g., downloaded data). Examples of services include human services (e.g., contracted work) and automated services (e.g., subscriptions). Other examples of products include authorizations (e.g., access to services, licenses, or encryption keys). In merchandising an item, the seller may use a network-based system to present an advertisement of the item to a user of the network-based system (e.g., a potential buyer of the item). Examples of network-based systems include commerce systems (e.g., shopping websites), publication systems (e.g., classified advertisement websites), listing systems (e.g., gift registries), transaction systems (e.g., payment websites), and social network systems (e.g., Facebook® or Twitter®). Examples of advertisements include a banner ad, a sidebar ad, a pop-up ad, a tool tip, a search result referencing the item, a listing for the item (e.g., within a list of items available for purchase), a review of the item, a comment on the item, or any suitable combination thereof.